What can pilots do to minimize parasite drag?
As far as I know the effect of parasite drag really depends on the manufacturer of the airplane, but can pilots do anything about it?
Parasite drag is for the most part out of a pilot's hands, as it's a combination of:
You can reduce skin friction drag by keeping the airplane clean, dirt and smashed bugs cause friction, removing them reduces friction. You can also make sure all the little doors are flush. Other than that you can't do much without making changes to the airplane itself, for instance:
Whether these are worth the cost depends on the airplane, Cessna 152/172s are very draggy to begin with because of their rivets and wing struts, you could dump thousands into additions for very little benefit. Other airplanes get measurable improvements in speed from a few seals here and there.
Pilots can help keep parasitic drag to a minimum by:
Some competitors at the National Championship Air races in Reno, Nevada, depending on class, will tape-over wing and fuselage seams and gaps. This is commonly seen in the Sport Class where general aviation, kit based, planes compete.